Rev. Cubana Fis. 36, 152 (2019) PARA F´ISICOS Y NO F´ISICOS FINDING NEW WORLDS: DIDIER QUELOZ AND MICHEL MAYOR’S NOBEL PRIZE IN PHYSICS 2019 ENCONTRANDO NUEVOS MUNDOS: LOS PREMIOS NOBEL DE F´ISICA 2019 DE DIDIER QUELOZ Y MICHEL MAYOR R. CARDENAS´ y Planetary Science Laboratory, Universidad Central “Marta Abreu” de Las Villas, Santa Clara, Cuba; [email protected] † corresponding author Recibido 20/11/2019; Aceptado 23/11/2019 PACS: Astrometry (astrometr´ıa), 95.10.Jk; Instrumentation (instrumentacion),´ 95.55.Br; Astrobiology (astrobiolog´ıa), 91.62.Fc, 96.55.+z I. INTRODUCTION if it is approaching), and c is the speed of light in vacuum. For the non-relativistic limit, it can be shown that, Since ancient times, humans have speculated on the potential V existence of other Solar Systems, with planets orbiting a parent z β = ; (3) star [1,2]. In 1952 a method based on the measurement of ∼ c stellar radial velocities was proposed to detect exoplanets [3]. which is the classical expression for the radial redshift caused However, formidable technical challenges remained a major by Doppler effect (we note that motion in the transverse obstacle for several decades. In 1992 –40 years later– the direction causes the so called transverse Doppler effect, so first observational detection of exoplanets was reported by far of little or no use for exoplanet detection). astronomers Aleksander Wolszczan and Dale Frail [4]. Using the Arecibo radio telescope, they detected two giant planets As follows from equations1 and2, measuring z would allow orbiting the pulsar PSR B1257+12 in the constellation of Virgo. calculating V, the velocity of the star. However, this is strictly Many astronomers were surprised, as they expected to find true only if the stars woobles in the line of sight of the observer such planets only around main sequence stars. on Earth, so the inclination i of the planet’s orbit respect to the line perpendicular to the line-of-sight should be considered: II. THE METHOD OF STELLAR RADIAL VELOCITIES TO DETECT EXOPLANETS V V = D ; (4) As mentioned above, the first method to detect exoplanets sin i was based on the measurement of stellar radial velocities. A where VD is the observed (Doppler) velocity of the star. Also, star and its exoplanet revolve around the centre of mass of the the period of motion of the star can be determined using system (the barycenter), as illustrated in Figure1. Doppler spectroscopy, while its mass is typically determined Due to the Doppler effect, an observer on Earth will receive using the well-known mass-luminosity relationship. Once the blue-shifted light when the star is moving towards him (or stellar properties are determined (radial speed V, period T her), and red-shifted light when the star is moving away from and mass M), it is possible to determine some properties of him (or her). Including relativistic effects, the redshift z can be the exoplanet. calculated as, First we may apply the third Kepler’s law (the square of the orbital period T of a planet is directly proportional to the cube s of the semi-major axis of its orbit r): λ0 λ 1 + β z = − = 1; (1) 3 GM 2 λs 1 β − r = T ; (5) − 4π2 where λ0 is the wavelength detected by the observer on Earth where G is the gravitational constant and, as both periods are and λs is the wavelength in the reference frame of the star, and equal, in5 it is used the observed period of the star, T. Having the parameter β is defined as: determined r, the velocity of the planet v around the star can be calculated using Newton’s law of gravitation and the orbit V β = ; (2) equation: c r where V is the radial velocity of the star relative to the observer GM v = : (6) on Earth (taken positive if the star is receeding, and negative r REVISTA CUBANA DE F´ISICA, Vol 36, No. 2 (2019) 152 PARA F´ISICOS Y NO F´ISICOS (Ed. E. Altshuler) Figure 1. Use of the Doppler effect to estimate the radial velocity of a star, as seen from Earth. Figure modified from [5]. Then, application of the conservation of linear momentum Someone monitoring our Solar System would observe a radial allows determining the mass m of the planet: velocity change of 13 m/s of the Sun over 12 years due to the ± r orbital motion of Jupiter [1]. Of course, this tiny variation MV imposes a severe challenge from the observational point of m = : (7) v view. Michel Mayor at the University of Geneva, Andre´ Baranne There are other methods to detect exoplanets. For instance, from the Marseilles Observatory and collaborators designed Transit Photometry, which uses the decrease of the intensity a new echelle spectrograph: the ELODIE instrument [6,7]. of star’s light when the planet passes through the line of sight Using it on a sample of 142 stars, and surveying around of an observer from Earth. Also Gravitational Lensing and 5000 spectral lines for Doppler spectroscopy, they found that other more recent methods [5]. the radial velocity of the star 51 Pegasi (in the Pegasus constellation), had variations with a period of about four III. A NEW EPOCH IN EXOPLANET DETECTION days (see Figure2). In 1995 Didier Queloz and Michel Mayor announced the discovery of 51 Pegasi b, the first detected exoplanet orbiting a main sequence star [8]. Because of this breakthrough discovery, they have been awarded the Nobel Prize in Physics 2019 (shared with Jim Peebles for theoretical findings in Cosmology; see the corresponding article in this issue). IV. IMPLICATIONS FOR ASTROBIOLOGY The first announcement of a confirmed exoplanet [4] was taken with caution by the Astrophysics community: several previous claims of discovery had been later rejected because of experimental noise. Additionally, theory predicted only a very small fraction of planets orbiting a pulsar (which has been confirmed late on). Thus, the discovery of Michel Mayor and Didier Queloz of the first exoplanet orbiting a Sun-like Figure 2. Position of 51 Pegasi b, the first exoplanet detected orbiting a main star opened a new door in Astrophysics and Astrobiology. sequence star. The sketch represents the sky over Stockholm in October. Five years after the discovery, when the first review “post–51 Figure adapted from [5]. Pegasi” appeared, 34 exoplanets had been discovered orbiting REVISTA CUBANA DE F´ISICA, Vol 36, No. 2 (2019) 153 PARA F´ISICOS Y NO F´ISICOS (Ed. E. Altshuler) Sun-like stars [1,2], and as of 7 November 2019, there are collaborations between astrobiologists and environmentalists 4093 confirmed exoplanets, and more than 3000 expecting for [12, 14–16]. confirmation [9]. An inmediate question arose: are exoplanets habitable? It ACKNOWLEDGEMENTS motivated a closer link between astrobiologists, planetary scientists and environmentalists. It also contributed to further R. Cardenas´ and E. Altshuler acknowledge the revision of this development of the quantification of habitability, with three manuscript by Nobel laureate M. Mayor. (complementary) approaches: the astrobiological one, the biogeochemical one and the ecological one. Based on metrics of quantitative habitability, there exists a catalog of potentially REFERENCES habitable exoplanets, maintained by the Laboratory of Planetary Habitability of the University of Puerto Rico [1] The Nobel Committee for Physics, Scientific Background at Arecibo [10]. So far, it acknowledges 55 potentially on the Nobel Prize in Physics 2019 (2019). habitable exoplanets using climatological criteria, especially the possibility of having liquid water at the planetary [2] M.A.C. Perryman, Rep. Progr. Phys. 63, 1209 (2000) surface [11]. However, it should be noticed that, in our [3] O. Struve, Observatory 72, 199 (1952) planet, it is estimated that the subsurface biosphere is [4] A. Wolszczan and D. A. Frail, Nature 355, 145 (1992) comparable in mass and volume to the surface one, a fact [5] https://en.wikipedia.org/wiki/Exoplanetology#Indirect % that multiplies the possibilities of existence of living entities 20methods Accessed 12.12.2019. in other planetary bodies. 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Lett. 787, L29 (2014) campaigns to name them [13], led by the International [12]R.C ardenas,´ D. Avila-Alonso, N. Perez-D´ ´ıaz, O. Astronomical Union (IAU). In Cuba, a commission of 5 Martin-Gonzalez,´ R. Nodarse-Zulueta, On the scientists is working to name the star BD-17 63 (an orange quantification of habitability: current approaches, dwarf star) and its exoplanet BD-17 63 b. This system is located Proceedings of the 2nd International Conference on at the constellation Cetus (the Whale) and, as ruled by IAU, is BioGeoSciences - Modeling Natural Environments, visible from Cuba, the naming country. (Springer-Nature, 2019), pp 1-8 [13] https://www.iau.org/public/themes/naming exoplanets/ VI. CONCLUSIONS [14]A.M endez,´ The First Billion Years: Habitability 2019, https://www.hou.usra.edu/meetings/habitability2019 The breaktrough Discovery of Didier Queloz and Michel /pdf/1033.pdf (2019) Accessed 12.12.2019.